BSE in the US is scary. The USDA tests less than 1% of the animals that go into the food supply (even though the test cost less than $5). Worse, when a US producer wanted to test 100% of his animals (to export to Japan) the USDA prevented him from doing it (
http://www.ens-newswire.com/ens/apr2004/2004-04-15-02.html ) on the basis that if he tested his animals, then people might loose faith in the other processors. Worse, if his animals from tightly controlled circumstanced came up positive, the entire US herd may actually have a certain percentage of positive animals that are undetected. That could kill exports.
Now, given that BSE is a prion disease that takes at least 2 years to become significantly evident, that fact that most positives in the US are in dairy is normal (they aren't slaughtered until later in life, so they show up at the plant symptomatic and requiring testing). What they don't mention is that the calves produced by that cow may have gone into the food supply, but didn't 'pop positive' because they weren't symptomatic (slaughtered at 18mo or less) and didn't fall into the 1% tested.
What I don't understand is how did this cow get access to BSE (a prions disease) of any kind if it wasn't through feed? Prions are thought to be nearly indestructable. Are they saying this cow picked them up on pasture? Or was she out wandering the bad part of town? In the past, they've accused a positive cow of having been shipped into the US from Canada in an already positive state. If this cow was born and raised in the US, then there is at least a form of BSE in the US that needs to be tracked down.